1. Field of the Invention
This invention relates to apparatus and method for encoding and decoding information in audio signals, such as those commonly recorded on records, tapes, and compact discs.
2. Description of Related Art
Audio codes have been used for centuries. Audio codes include the use of jungle drums to communicate information. Defined broadly, audio codes even include human speech. In the modern age audio codes have included the transmission of morse code, in the form of tones of varying length, over the airwaves. Such codes function well when conveying information to humans and have served useful functions.
In this modern age in which electronics are making it possible for machines to perform an ever increasing number of functions, it is desirable to combine machine readable codes with an audio signal designed for human listening. For example, audio cue tones have been placed on audio tapes to help a tape player advance to and stop at the location at which the tone occurs. The problem with such tones is that their presence can interfere with the enjoyment of listening to audio signals by human listeners, and they do not carry very much information.
It is also known to pulse-width modulate a signal to provide a common or encoded signal carrying at least two information portions or other useful portions. In U.S. Pat. No. 4,497,060 to Yang (1985) binary data is transmitted as a signal having two differing pulse-widths to represent logical "0" and "1" (e.g., the pulse-width durations for a "1" are twice the duration for a "0"). This correspondence also enables the determination of a clocking signal.
Various techniques for encoding signals are also known. For example, U.S. Pat. No. 4,937,807 to Weitz et al. (1990) discloses a method and apparatus for encoding signals for producing sound transmissions with digital information to enable addressing the stored representation of such signals. Specifically, the apparatus in Weitz et al. converts an analog signal for producing such sound transmissions to clocked digital signals comprising for each channel an audio data stream, a step-size stream and an emphasis stream. The device and method also include editing the encoded digital signals to add other information to enable high volume storage, direct access and higher throughput.
With respect to systems in which audio signals produce audio transmissions, U.S. Pat. No. 4,876,617 to Best et al. (1989) and U.S. Pat. No. 5,113,437 to Best et al. (1992) disclose encoders for forming relatively thin and shallow (e.g., 150 Hz wide and 50 dB deep) notches in mid-range frequencies of an audio signal. The earlier of these patents discloses paired notch filters centered about the 2883 Hz and 3417 Hz frequencies; the later patent discloses notch filters but with randomly varying frequency pairs to discourage erasure or inhibit filtering of the information added to the notches. The encoders then add digital information in the form of signals in the lower frequency indicating a "0" and in the higher frequency a "1". In the later Best et al. patent an encoder samples the audio signal, delays the signal while calculating the signal level, and determines during the delay whether or not to add the data signal and, if so, at what signal level. The later Best et al. patent also notes that the "pseudo-random manner" in moving the notches makes the data signals more difficult to detect audibly.
An area of particular interest to certain embodiments of the present invention relates to the market for musical recordings. Currently, a large number of people listen to musical recordings on radio or television. They often hear a recording which they like enough to purchase, but don't know the name of the song, the artist performing it, or the record, tape, or CD album of which it is part. As a result, the number of recordings which people purchase is less than it otherwise would be if there was a simple way for people to identify which of the recordings that they hear on the radio or TV they wish to purchase.
Another area of interest to certain embodiments of the invention is copy control. There is currently a large market for audio software products, such as musical recordings. One of the problems in this market is the ease of copying such products without paying those who produce them. This problem is becoming particularly troublesome with the advent of recording techniques, such as digital audio tape (DAT), which make it possible for copies to be of very high quality. Thus it would be desirable to develop a scheme which would prevent the unauthorized copying of audio recordings, including the unauthorized copying of audio works broadcast over the airwaves.
The prior art fails to provide a method and an apparatus for encoding and decoding analog audio frequency signals for producing humanly perceived audio transmissions with signals that define digital information such that the audio frequency signals produce substantially identical humanly perceived audio transmission prior to and after encoding. The prior art also fails to provide relatively simple apparatus and methods for encoding and decoding audio frequency signals for producing humanly perceived audio transmissions with signals defining digital information. The prior art also fails to disclose a method and apparatus for limiting unauthorized copying of audio frequency signals for producing humanly perceived audio transmissions.